1905. | On Vegetable Assimilation and Respiration. 457 
incident on a leaf, it requires special investigation to determine whether the 
effect is actually due to the increase of light or to the increase of 
temperature. 
Single leaves of cherry-laurel or Helianthus were enclosed in a glass 
chamber sunk in a glass water-bath, of which the temperature could be 
controlled, and hourly or half-hourly estimations made of their assimilation 
under all varieties of natural illumination. The exact temperature of the 
leaf was determined thermo-electrically all through the experiments, and the 
illumination was either the natural diurnal sequence in the open air or 
selected illumination, eg., diffuse light alone, full sunshine alone, or definite 
fractions of sunshine. The current of air through the leaf chamber was 
enriched with CO,, so that inadequate supply of this gas might never be a 
limiting factor. 
To test whether an observed assimilation-value is limited by light or by 
temperature, it suffices to change the intensity of each factor separately. 
Thus, in the case that temperature is limiting, circulation of hotter water 
through the bath sends up the assimilation without any increase of illumina- 
tion, while, if the temperature is carefully kept constant, the illumination 
can be increased to any extent without causing a rise of assimilation. 
Assimilation in shade and in sunshine was analysed in this way. 
With a leaf exposed to diffuse daylight only, the amount of assimilation 
is a measure of the light if the temperature is kept high, and then the 
assimilation will fall all through the afternoon with the waning light. If 
the temperature is low, then assimilation is thus limited, and remains 
uniform all through the varying light. Real assimilation persists until 
sunset. The diffuse light may have a very high photosynthetic intensity 
when the sky is covered with white reflecting cumuli or with a thin haze. 
With a leaf kept exposed normally to the sun throughout the day a very 
rapid rise of assimilation sets in at sunrise, but the amount of assimilation 
is generally limited by the temperature, since the illuminating effect of the 
sun exceeds its heating effect, especially when the leaf is included in a bath 
of circulating water. 
To apply to assimilation in free air the data obtained with a leat in a 
water-bath, it is necessary to know the actual internal temperatures that 
leaves attain in sun and shade in the open air. Thermo-electric measure- 
ments with leaves of cherry-laurel, exposed normally to brilliant insolation, 
showed that the internal temperature may rise more than 10° above that 
registered by a bright mercury thermometer in the sun: a leaf exposed in a 
closed glass vessel may be heated a further 10°. 
At natural temperatures neither Helianthus nor cherry-laurel is capable 
